1. Field of the Invention
The present invention relates to an improved process for separating and recovering an organic phase (e.g., fluorocarbons) and hydrogen fluoride (HF) from a fluorocarbon/HF mixture, wherein because of the presence or potential formation of an azeotrope or azeotrope-like composition, it is impractical to separate the mixture by conventional distillation. More specifically, the present invention relates to separating a fluorocarbon/HF mixture, by use of a semipermeable membrane unit, into a HF depleted stream and a HF enriched stream that are then further processed individually by distillation.
2. Description of Related Art
It is well known that fluorochemicals of commercial interest including chlorofluorocarbons (CFC), hydrogen-containing chlorofluorocarbons (HCFC), hydrogen-containing fluorocarbons (HFC) and perfluorocarbons (FC) are typically manufactured by processes involving halogen exchange reactions. Generally the appropriate chlorocarbon is reacted with a fluorine-containing compound which serves as a fluorine donor. Most generally, the fluorine-donating source is hydrogen fluoride used in the presence of various catalytic compound. Such a process may be illustrated by the preparation of monochlorodifluoromethane (HCFC-22) wherein chloroform is the chlorocarbon employed and hydrogen fluoride is the fluorine source according to the following reaction: ##STR1##
The catalysts useful in this reaction include various metal oxides and halides and the reaction can be carried out in either vapor or liquid phase. The amount of hydrogen fluoride used in the above process is almost always in excess of the stoichiometric amount required and may be as much as a ten-fold excess. Excess hydrogen fluoride is used to increase yields and conversions and to reduce the reaction time. In the process illustrated by the above equation the crude reaction stream may contain some unreacted HCCl.sub.3, underfluorinated HCCl.sub.2 F, the desired HCClF.sub.2, the by-product HCl and the unreacted HF. By a combination of known processes such as distillation, phase separation and the like, hydrogen chloride can be recovered as useful anhydrous and/or aqueous hydrogen chloride, HCCl.sub.3 and HCCl.sub.2 F can be recovered for recycling as well as most of the hydrogen fluoride can be recovered by distillation processes. However, the desired product, HCClF.sub.2 (HCFC-22), cannot be recovered free of hydrogen fluoride by ordinary distillation since HCFC-22 and HF form an azeotrope-like mixture (b.p. 50.degree. C. at 292.5 psia and essentially constant composition of 97 wt. % HCFC-22 and 3 wt. % HF).
Various procedures have been proposed to recover HCFC-22 from its azeotropic mixture with HF, but these procedures introduce many undesirable problems. For example, an azeotropic mixture of HCFC-22 and HF can be washed with an aqueous alkali solution to convert HF into fluoride salt and thereby separate it from HCFC-22. HCFC-22 then requires dehydration and subsequent purification by distillation. The hydrogen fluoride neutralized by the alkali solution must be disposed of, creating disposal problems as well as representing loss of possibly recyclable reactant, thus having adverse economic effects.
Another possible procedure for recovering HCFC-22 from its azeotropic mixture with hydrogen fluoride is to contact the mixture with a concentrated sulfuric acid (as disclosed in U.S. Pat. No. 3,873,629) thereby to dissolve the hydrogen fluoride selectively in the sulfuric acid solution. Such a procedure also requires the additional steps of washing the thus separated HCFC-22 for the removal of the sulfuric acid, dehydration and then distillation. While the hydrogen fluoride dissolved in the concentrated sulfuric acid can be recovered for recycling by heating the sulfuric acid solution, such a recovery step requires specialized equipment and conditions such as are used in the hydrogen fluoride manufacturing process.
Van Eijl in U.S. Pat. No. 3,947,558 has suggested that hydrogen fluoride can be separated from an organic mixture of fluorinated C.sub.1 -C.sub.3 compounds and recovered by selectively absorbing the HF in a glycol in which the HF is soluble, but the fluorinated organic compound is substantially insoluble. This process, as suggested by Van Eijl, depends upon very selective solubility differences between the fluorinated compounds and hydrogen fluoride. As exemplified by Van Eijl, even when the fluorinated compound is a perhalo compound, there is a fair degree of solubility of the perhalo compound in the glycol and a fair degree of solubility of the glycol in the perhalo compound. When the fluorinated compound is a hydrogen-containing chlorofluoro compound, such as HCFC-22, the solubility of HCFC-22 in the glycol is enhanced as well as the solubility of HF in the glycol-containing HCFC-22 layer and thus the separation process is adversely affected.
It has also been previously recognized that HF can be separated from certain non-azeotrope systems by selective permeation through a membrane. For example, Grote in U.S. Pat. No. 4,661,296 discloses separation of HF from a carbonylation process mixture (e.g., isobutyric acid, water and HF) using a "NAFION" membrane. The disclosed process is an alternative to separation by distillation. Also, Tarasenko in U.S. Pat. No. 4,424,067 discloses purification of anhydrous HF by removal of AsF.sub.3 contaminant by permeation of the HF through non-porous fluoropolymer film. Again no separation of an azeotrope is suggested.